It is well known in the art that the physical properties of sputtering targets employed for Physical Vapor Deposition (PVD) in the electronics industry greatly influence the final properties of the thin films produced. In fact the target properties that enable and enhance the manufacture of high quality thin film devices and circuitry are:
Fine and uniformly fine grain structure.
Random and uniformly random crystallographic orientation of the individual grains.
A microstructure that when viewed on the macroscale is substantially invariant throughout the entire body of the target.
A microstructure that can be repeated from target to target.
A microstructure that is substantially 100% dense and provides high strength intergranular bonding.
These properties are in particular very difficult to attain in tantalum (Ta) and niobium (Nb) targets. This results from the fact that high purity Ta and Nb are refined and purified via electron beam melting and casting into a cold, water cooled mold. The ingot formed has many extremely large grains measured in multiples of centimeters in both width and length. These extremely large grains require extensive and expensive thermomechanical processing in order to reduce the grain size and reduce the crystallographic alignment of the individual grains (reduce the texture). Thermomechanical processing has limits in the reduction in grain size, crystallographic randomness produced, and the uniformity of microstructure produced. Typically tantalum target material produced from ingot still contains a large degree of nonuniformity described as grain size and texture banding-regions where there is a common grain size and texture that is not typical of the overall grain size and texture of the entire target.
The importance and magnitude of this problem was addressed in U.S. Pat. No. 6,193,821 where ingots were first side forged or side rolled followed by upset forging or upset rolling. U.S. Patent Publication No. 2002/0112789 A1 describes a process utilizing upset forging followed by draw back forging then side forging and finally a cross rolling process to provide a mix of grains with the {100} and {111} orientation. In U.S. Pat. Nos. 6,331,233 and 7,101,447 the inventor specified a complex three step process consisting of multiple deformation and annealing components. But while the complex processing route successfully refined grain size the processing still resulted in a predominantly {111} texture.
US Patent Publication No. 2005/0155856 A1 describes a Ta sputtering target which has a preferential (222) orientation over a limited portion of the target thickness which it is claimed improves the uniformity of the sputtered film thickness.
Other patents recognize the inherent advantages of starting with tantalum metal powder rather than a solid tantalum ingot. U.S. Pat. Nos. 5,580,516 and 6,521,173 describes cold compact Ta powder into billets that then may undergo a wide range of thermal/mechanical process techniques in order to produce fully dense billets from which sputtering targets can be made. U.S. Pat. No. 6,770,154 describes consolidating a powder billet to full density followed by rolling and annealing to provide a uniform but not random grain structure. U.S. Pat. No. 7,081,148 expands upon the processes of U.S. Pat. No. 6,770,154 to include a resultant tantalum sputtering target that is at least 99.99% pure tantalum.
U.S. Pat. No. 7,067,197 describes a powder metallurgy process that first surface nitrides the tantalum powder before compaction. The surface nitride powder may then be compacted by a list of at least 23 different processing steps that must retain the high nitrogen content of the powder. One of the least favorable is spray depositing, although no mention of what type of spray deposition technology is being used i.e. plasma spray, low pressure plasma deposition, flame spray, high velocity oxyfuel, etc. a few of the many processes currently employed.
WO 2006/117145 and WO 2006/117144 describe cold spray processes for producing coatings of tantalum.
The rejuvenation or reprocessing or repair of used targets is also of economical interest due to the fact that tantalum and the processes for bonding tantalum to backing plates are quite expensive. This is compounded by the fact that only about 25-30% of a planar target and 60-70% of a rotary target is used in sputtering before the entire target must be replaced. Thus the recovery of the unused Ta is of much interest.
U.S. Patent Publication No. 2004/0065546 A1 discloses a method of hydriding the tantalum target so that the tantalum is embrittled allowing it to be separated from the backing plate, ground up, and reused as a powder stock in making ingots. U.S. Patent Publication No. 2006/0032735 discusses the use of laser beams and other focused energy sources in order to simultaneously melt and fuse powder that is fed into the worn areas of a used target in order to fill the void created by the sputtering. Of course all these techniques generate substantial heat and require the backing plate be removed from the target prior to repair. Additionally, as is well known to one of ordinary skill in the art, when melting occurs the powders resolidify by directional manner and the resulting microstructure has strong textural components.
Before a target can be used it must be machined to final dimensions and then soldered, brazed or diffusion bonded to a high thermal conductivity backing plate for mounting in the sputtering machine.
Sputtering targets are used to make a wide range of thin films with applications ranging from reflective and low emissivity coatings for window glass (Nb), photovoltaic films (Mo), narrow pass filters (TaNb) etc. Perhaps their best known use however is in integrated circuitry where layered sputtered films are used to make the basic switching devices as well as the circuitry to connect them producing functional electronic components (integrated circuits, flat panel displays, etc.). As stated above the quality of the thin films made and hence the quality of the products made using thin film technology, are highly dependent on the quality of the target they are sputtered from.
Cold spray or kinetic spray (see U.S. Pat. Nos. 5,302,414, 6,502,767 and 6,759,085; Van Steenkiste et al, “Analysis of Tantalum Coatings Produced by the Kinetic Spray Process” Journal of Thermal Spray Technology, Vol. 13 (2) June 2004 pages 265-273, U.S. Pat. No. 6,139,913, and U.S. Publication Nos. 20050120957 and 20050252450) is an emerging industrial technology that is being employed to solve many industrial manufacturing challenges (see, also e.g., U.S. Pat. Nos. 6,924,974; 6,444,259; 6,491,208 and 6,905,728).
Cold spray employs a high velocity gas jet to rapidly accelerate powders, typically less than approximately 44 microns in size, to high velocity such that when they impact a surface the powders bond to the surface to form an integral, well bonded and dense coating. The cold spraying of tantalum powders onto a variety of substrates (including steel) has been suggested (see, e.g., “Analysis of Tantalum Coatings Produced by the Kinetic Spray Process,” Van Steenkiste et al, Journal of Thermal Spray Technology, volume 13, number 2, June 2004, pages 265-273; “Cold spraying—innovative layers for new applications,” Marx et al, Journal of Thermal Spray Technology, volume 15, number 2, June 2006, pages 177-183; and “The Cold Spray Process and Its Potential for Industrial Applications,” Gartner et al, Journal of Thermal Spray Technology, volume 15, number 2, June 2006, pages 223-232). This is all accomplished without having to heat the powder to a temperature near or above its melting point as is done with traditional thermal spray processes. The fact that dense coatings can be formed at low temperatures present many advantages. Such advantages include lack of oxidation, high density deposits, solid state compaction, the lack of thermally induced stresses and particularly, in this case, the lack of substantial substrate heating.
Kinetic spray can be accomplished for example, by injecting Ta starting powders having particle diameters great than 65 μm into a de Laval-type nozzle, entrained in a supersonic gas stream and accelerated to high velocities due to drag effects. The particle's kinetic energy is transformed via plastic deformation into strain and heat on impact with the substrate surface. The particles are melted in the process.
Limited substrate heating is preferred in the instance of manufacturing cathode or electronic sputtering target blanks for the field of Physical Vapor Deposition (PVD). Target materials are frequently high melting temperature (“TM”) refractory metals (Ta TM=2998 C) while the backing plate that supports the target is chosen for its high thermal conductivity and is typically copper or aluminum (Al TM=660 C), both low melting temperature materials. Thus other thermal spray processes that require heating of the powder to at or near its melting point can not be used to deposit refractory metals on the low melting temperature backing plate. Current practice is to make the target completely separate from the backing plate and then use solder, brazing, diffusion bonding or explosive bonding techniques in order to bond the target and backing plate together. Because cold or kinetic spray does not substantially heat the powder it can be used to make targets directly on the backing plate as well as repair used targets without the need of having to remove the target from the backing plate.